The present disclosure relates to three-dimensional measurement techniques of producing positional information of an object by emitting light and receiving its reflected light.
There is a three-dimensional measuring method based on the fact that the time of flight (TOF) from transmission of light pulses to reception of the pulses reflected from an object depends on the distance (hereinafter this method is referred to as the “TOF method”). Distance measurement (hereinafter referred to as “ranging”) using the TOF method has a problem that ranging accuracy is reduced by reflectance of a target object or by the surrounding environment.
Japanese Unexamined Patent Publication No. H10-332827 describes a method that suppresses such reduction in ranging accuracy. In this method, pulsed light is repeatedly emitted at regular intervals. The amount of each reflected pulsed light is converted to a fixed value, and the resultant reflected pulsed light is allowed to enter an imaging area sensor. The exposure amount in a predetermined period is proportional to the number of reflected pulsed lights (the number of pulses). The longer the time of flight is, namely the longer the distance is, the smaller the exposure amount is. Since the amount of each reflected pulsed light is converted to the fixed value, three-dimensional measurement independent of the reflectance of the target object can be implemented.
Japanese Unexamined Patent Publication No. 2001-337166 describes another method that suppresses such reduction in ranging accuracy. In a long distance mode, light is emitted with a uniform luminous intensity distribution, and ranging is performed by using the TOF method. In a short distance mode, light is emitted with two different luminous intensity distributions, and the ratio between incident light intensities is obtained. In the short distance mode, ranging is performed by using a triangulation method. This method can implement accurate three-dimensional measurement in a larger distance range.
In the case of measuring the distance to a target object by using the TOF method, the target object is irradiated not only with direct light beams from a light source unit but also with optical multipath resulting from scattering of other direct light beams by the surrounding environment such as a wall (hereinafter the optical multipath is referred to as the “scattered light”). A light receiving unit receives not only reflection of the direct light beams (hereinafter referred to as the “direct reflected light”) but also reflection of the scattered light (hereinafter referred to as the “unwanted reflected light”). In this case, the light receiving unit generates electric charge corresponding not only to the direct reflected light component but also to the unwanted reflected light component. Depending on the surrounding environment, this may significantly reduce ranging accuracy in the three-dimensional measurement of the target object.
If there is a mirror in an irradiation region, a large amount of unwanted reflected light is generated, which causes a significant measurement error in a distance image of the target object.
In Japanese Unexamined Patent Publication Nos. H10-332827 and 2001-337166, no measures are taken against the measurement error due to the influence of the unwanted reflected light, and the influence of the unwanted reflected light cannot be eliminated.
In view of the above problems, it is an object of the present disclosure to remove or reduce an unwanted reflected light component generated by the surrounding environment in three-dimensional measurement using a TOF method and implement accurate three-dimensional measurement.